Quantum unitary evolution is time symmetric, what leads to retrocausality phenomenas like delayed choice quantum erasure or Wheeler's experiment. However we understand why they don't allow to send information back in time - I would like to ask for help with understanding why another experiment also does not allow for that: As physics we use in all scales is time or CPT symmetric Lagrangian mechanics like QFT, let us imagine CPT analogue of laser (lasar: stimulating absorption instead of excitation), like the conceptually simplest free electron laser (FEL): https://dl.dropboxusercontent.com/u/12405967/freeelectron.jpg [Broken] In standard FEL we use magnetic field to enforce electrons to travel in sinus-like pattern, what stimulates photon emission due to synchrotron radiation. These photons are later absorbed by the target, exciting it. Let us now imagine CPT analogue of this situation: excited target deexcitates - producing photons finally absorbed by positron moving in opposite direction ... but the situation is stimulated by the sinus-like trajectory - we get stimulated absorption setting instead of standard stimulated emission. So imagine we constantly excite the target (e.g. a sodium lamp), surround it with detectors - we should observe energy conservation. Now if there is a small hole in detectors to the FEL in stimulated absorption setting (oppositely directed) and set to the same frequency as the excitement (it is rather not fulfilled in standard settings) - shouldn't turning it on change the lamp-detectors energy balance?